1. Field of the Invention
The present invention relates to an inverter for use with a winding system and a method for controlling the inverter which controls the speed of a motor driving a reel of the winding system that winds a work in the form of an elongated material on the reel at a certain speed to form a coil.
2. Description of Related Art
FIG. 1 shows a typical conventional winding system. In this figure, the reference numeral 1 designates a winding shaft of a reel on which a work 3, e.g., an elongated material such as wire is wound to form a coil 2. The reference numeral 4 designates an AC motor for driving the reel, 5 denotes a dancer roll, 6 designates a detector for detecting the displacement of the dancer roll 5, and 7 denotes a feeder of the work 3.
The speed of the motor 4 is controlled by an inverter 20 comprising an initial primary speed setting portion 11, a summing and PI control portion 13, and a PWM control portion 14. The initial primary speed setting portion 11 determines the initial peripheral speed V.sub.0 of the coil 2 (that is, the initial winding speed of the reel) at the start of the winding operation. The summing and PI control portion 13 adds or subtracts a speed correction amount .DELTA.D to and from the initial primary speed V.sub.0 in accordance with the polarity of the correction amount .DELTA.D, and performs the PI operation on that result to determine the output frequency f.sub.i of the inverter 20. Here, the correction amount .DELTA.D is obtained by converting the output of the detector 6 that detects the displacement of the dancer roll 5. This displacement corresponds to the looseness or tension of the work 3. The correction amount .DELTA.D is zero at the start of the winding operation because the initial winding speed equals the initial primary speed V.sub.0.
In the winding system as shown in FIG. 1, the peripheral speed of the coil 2 will increase with the build-up of the radius of the coil 2 as the winding operation proceeds if the rotational speed of the reel is maintained constant. On the other hand, the peripheral speed of the coil 2 (that is, the winding speed) must be equal to the line speed (that is, the work feed speed) because the feeding rate of the work 3 is constant. This requires the correction of the speed of the motor 4 in response to the build-up of the radius of the coil 2.
To achieve this correction, the conventional winding system detects the displacement of the dancer roll 5 or the fluctuation of the tension of the work 3, and produces the correction amount on the basis of the detected value. More specifically, the speed of the motor 4 for driving the reel is controlled as follows: First, the initial output frequency of the inverter 20 at the start of the winding operation is set at a value corresponding to the rotational speed of the motor that provides the initial winding speed of the reel. Second, the output frequency of the inverter 20 is corrected in accordance with the difference between the line speed at which the work 3 is transferred and the reel winding speed, that is, the peripheral speed of the coil 2, thereby maintaining the peripheral speed, which might increase consistently with the diameter build-up of the coil 2, equal to the line speed. The correcting control is carried out by detecting the displacement of the dancer roll 5 placed in the way of the winding path for absorbing looseness of the work 3, or by detecting the fluctuation of the tension of the work 3. For example, when the dancer roll 5 moves downward owing to an increase in the looseness of the work 3, the output frequency of the inverter 20 is increased so that the speed of the motor 4 is increased.
FIGS. 2A and 2B are graphs illustrating the change in the correction amount .DELTA.f of the output frequency of the inverter 20. In these figures, the point P indicates the starting point of the winding operation, and the output frequency f.sub.i of the inverter 20 is represented as a function of the radius R of the coil 2. The difference .DELTA.f.sub.k (k=1, 2, ...) is the frequency equivalent of the correction amount .DELTA.D associated with the displacement of the dancer roll 5.
FIG. 2A shows the behavior of the output frequency f.sub.i while the radius R of the coil 2 varies from the initial radius R.sub.ii to the final radius R.sub.if during the winding process. According to such change in the radius R of the coil 2, the output frequency f.sub.i of the inverter undergoes the frequency reduction correction which is performed by subtracting the correction amounts .DELTA.f.sub.1, .DELTA.f.sub.2, .DELTA.f.sub.3, etc. from the initial value fii corresponding to the initial peripheral speed V.sub.0 of the coil 2, and reaches the final value f.sub.if. Here, the characteristic curve L denoted by the dashed-and-dotted line represents a virtual curve that would be obtained when the output frequency f.sub.i of the inverter 20 is reduced in inverse proportion to the radius R of the coil 2. In practice, however, the amount of correction in accordance with the displacement of the dancer roll 5 will oscillate in a narrow region across the settling values determined by the inertia of the winding control system. The characteristic curve L corresponds to the average of the fluctuation.
FIG. 2B is an enlarged graph showing the portion A of FIG. 2A to illustrate the oscillation behavior of the frequency reduction correction amount .DELTA.f.sub.k. As shown, the output frequency f.sub.i of the inverter 20 oscillates across the characteristic curve L.
Thus, in the conventional winding system, the correction amount for reducing the output frequency of the inverter 20 increases as the winding operation proceeds. As a result, if the gain for the correction amount with regard to the initial primary speed is increased in order to increase the response speed of the winding control, both the displacement of the dancer roll 5 and the correction amount associated with the displacement will increase at the same time. This will increase the fluctuation amount of the dancer roll 5 in the vertical direction and degrade its settling characteristic in a case where some disturbance occurs or the winding operation is restarted after an interruption.
In other words, the conventional winding system cannot achieve a reliable, accurate winding operation. This may deteriorate the quality of the coil 2, a product which is formed by winding the work 3. Furthermore, a large space must be provided for allowing the vertical movement of the dancer roll 5.